import numpy as np
import matplotlib.pyplot as plt

x = np.array([0, 1, 2, 3])
y = np.array([0, 1, 2, 3])

# z = np.array([[0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0]])
z = np.array([[1, 1, 1, 1],
              [1, 1, 2, 2],
              [1, 2, 2, 2],
              [2, 2, 2, 2]])
plt.contour(x, y, z, levels=[1.5], alpha=0.3, cmap='viridis')
plt.show()
